In an environment where there are front and back seats like an interior of an automobile, sounds generated at the front seat are difficult to be transmitted to the back seat due to the sound directivity. As a result, it is difficult to have a conversation. In addition, during driving, it becomes more difficult to have the conversation due to the masking effect of driving sound noise. In order to solve such problems, there is a technique for assisting conversations by a microphone and a speaker. According to this technique, in order to perform sound enhancement, sound collection by a microphone is performed at a location where the SN ratio (signal-to-noise ratio) is higher compared to that for the back seat, and an output by a speaker is performed near the back seat. Such system in a small room like the interior of the automobile brings about a closed-loop system, and thereby a howling or an echo occurs by a speaker output collected by the microphone. Hence, in general, a howling canceller, an echo canceller, etc., are mounted. However, those cancellers are for suppressing the howling or the echo in an output signal, but are not for suppressing reverberation in a sound field which is mixed in the room. In a small room, an original sound signal (i.e. an audible sound without passing through the microphone) is also present. Therefore, if a speaker output is mixed with the original sound signal, a sense of reverberation occurs in a room sound-field even when the speaker output itself does not include reverberation.
In addition, when a speaker output is performed from the back with large in sound level, the original sound from the front cannot be heard, impairing front localization. When the speaker output is delayed, front localization can be obtained due to the Haas effect. However, it causes a sense of reverberation, resulting in a double sound depending on the amount of delay.
For dealing with the foregoing problems, there is a Patent Literature 1 that discloses an echo canceller that determines a call state in a hands-free call, a telephone conference system, etc., to change a parameter, and performs echo cancellation according to the circumstances. In addition, there is a Patent Literature 2 that discloses a howling suppression device that suppresses reverberation in a field by performing frequency band division for a speaker output under an environment where reverberation occurs, to adjust the gains of adjacent frequency bands.
Patent Literature 1: JP 2009-021859 A
Patent Literature 2: JP 2010-151965 A